1. Field of the Invention
The invention relates generally to programmable logic devices, and more particularly to a programmable logic device having on-chip analog functional capabilities.
2. Description of the Related Art
Programmable logic devices (PLD), including the programmable array logic (PAL) device and the programmable gate array, are integrated circuits which can be configured by the user to perform logic functions on digital inputs. PLDs offer digital designers a flexible and cost-effective implementation for complex logic circuits and the best alternative amongst a spectrum of products ranging from fully customized integrated circuits to standard, dedicated-purpose devices.
A typical PAL includes a programmable array of AND gates, and a fixed array of OR gates. In other programmable logic devices, both the AND and the OR arrays are programmable. The outputs of such combinatorial logic arrays used in programmable logic circuits may be coupled directly to an I/O pin, or input to clockable registers. In many devices, the combinatorial logic array outputs are registered and are fed back to the inputs of the combinatorial array. Some programmable logic circuits also include a clockable input synchronizing register located between an input pin and one of the inputs to the combinatorial array. PAL is a trademark of Advanced Micro Devices, Inc. See, Advanced Micro Devices, Inc. PAL.RTM. Device Handbook, (1988).
The programmable logic device has thus given the digital designer a means for reducing circuit size through higher integration, ease of design and documentation by software specification, and the security of keeping the design proprietary.
It is desirable to provide a device which allows a user the ability to perform programmable output functions on variable amplitude inputs. Part of the reason for the lag in development between the analog and digital worlds is that designers typically try to accomplish analog functions by performing an analog-to-digital conversion at the earliest possible point in the data signal path to implement the solution in the digital realm, while performing a digital-to-analog conversion as the final step before outputting the signal.
A single integrated circuit having both analog and digital functional components working together to accomplish a programmable function possesses great flexibility and programmability by allowing programmable digital solutions to be derived from analog inputs.
One relatively simple programmable/analog type device is the X9MME E.sup.2 POT Digitally Controlled Potentiometer by Xicor Corporation, Milpitas, California, which provides a 99-increment resistor ladder controlled by a digitally decoded counter, providing a variable resistance load based on the count value. The device includes the capacity to store a count value in non-volatile memory on power down.